1. Field of the Invention
This invention relates to a transient spectroscopic method and apparatus for in-process analysis of molten metal to determine the elemental composition of the molten metal.
2. Discussion of Background
Determination of the elemental composition of molten metals as one of the process control data requires that the measurement be of real-time, in situ nature. The successful technique must be able to overcome most variations that exist of the thermal and fluid dynamic state of the molten metal and of the chemical properties of the slag layer above. Furthermore, the technique cannot rely on any physical phenomena which depend sensitively on the physical properties of the molten metal such as shear viscosity, surface tension, elemental vapor pressure and sound speeds. Of course, any sensor elements employed in the technique must either be able to survive the bath temperature or be provided with cooling without risking freeze-up of the slag or molten metal on them.
These requirements eliminate virtually all but the two following approaches: (a) excitation and subsequent spectroscopic examination of the particulate and gaseous effluents from the molten metal bath and (b) rapid vaporization and atomic excitation of a vapor plume from a slag-free molten metal surface by intense laser pulses, followed by spectroscopic analysis of the emission spectra. The first approach is advantageous in that measurement activities may be taken outside of a given furnace, thus allowing for extensive instrumentation, and is intrinsically well suited for generating a great deal of process data. It is, however, burdened with the need to determine under real-time conditions the extent of elemental contributions to the effluents by the slag in relationship to the molten metal. Prior investigation into the mechanisms of particulate production in steel furnaces has shown that the relative contributions depend strongly on the intensity of gas bubbling in the molten metal bath and the furnace temperature profile as well as the slag composition and the nature of nucleation centers for particulates (see T. W. Harding et al., "Direct Sampling of Gas and Particulates from Electric Arc Furnaces", in Proceedings of APS/AISI Conference on Physics in Steel Industry, Lehigh University, 1981. American Institute of Physics Conference Proceedings No. 84 (1982), pp. 362-376, and J. R. Porter et al., "Characterization of Directly Sampled Electric Arc Furnace Dust", Proceedings of APS/AISI Conference on Physics in Steel Industry, Lehigh University, 1981, American Institute of Physics Conference Proceedings No. 84 (1982), pp. 337-393.). While there are ways to meet this need, it is clear that extensive research must be carried out.
Examples of the first approach above outlined are discussed in U.S. Pat. No. 4,730,925 to Chiba et al and Frazer, "Continuous Monitoring of Melt Composition", NASA Tech Brief, Vol. 8, No. 2, Item No. 34, 1983.
Various attempts reported in the patent literature have been made to implement the second approach above noted. In U.S. Pat. No. 4,578,022 to Kenney, there is disclosed an apparatus for generating an aerosol powder from a metal melt. According to Kenney, a probe having an atomization dye is partially immersed in the metal melt so that the melt passes through an orifice in the atomization dye to create an aerosol powder. The aerosol powder is then transported by an inert gas to an inductively coupled plasma torch remote from the probe where the metal powders are heated and excited to emit atomic spectra characteristic of their constituent elements. However, the production of the aerosol powders skews the compositional distribution of the elements within each aerosol powder particle because the high evaporation elements are driven out by evaporation if the carrier gas is still hot. Further, all the aerosol particles are not the same size, with a result that the smaller the size of each aerosol particle, the larger the surface area to volume ratio of that particle. Therefore, the particles actually analyzed typically are those aerosol particles which exhibit the effect of more evaporation because many of the larger particles are lost by sedimentation, i.e., sticking to the walls during transport. Thus, the chemical composition of the molten metal is not accurately represented by the chemical composition of the aerosol powders subjected to analysis. Further, transportation of the aerosol powders to remote processing increases the likelihood of contamination of the aerosol powder. Subjection to a remote plasma flame can potentially compound the problem of contamination, so that the spectroscopic measurement subsequently performed are inherently inaccurate.
U.S. Pat. No. 4,598,577 to Jowit teaches laser ablation and evaporation of a molten metal by means of a laser housed in a probe which is immersed into the molten metal. Accordingly to Jowit, all or part of the vaporized metal is transported to a remote analytic apparatus including a plasma torch for heating the vaporized metal and a spectrograph for spectroscopic analysis of the plasma produced by the plasma torch. However, transporting of vaporized metal which is recondensed into particulates suffer from the same loss of larger particulates and preferential evaporation, which skews the subsequent spectroscopic analysis.
In British Patent No. 2,154,315A to Spenceley et al, on the other hand, a portion of a metal melt is excited by means of a pulsed laser beam and the radiation transmitted from the excited metal melt is transmitted through a light guide to an off line spectrometer for spectroscopic analysis of the spectrum produced by the excited portion of the melt. This technique, however, suffers due to the fact that a considerable amount of radiation from the excited melt does not enter the light pipe, and there is considerable absorption of this radiation during transmission via the light pipe to the analysis equipment. Further, this absorption of the radiation by the light pipe varies as a function of frequency, and thus impedes accurate measurement of the relative amplitudes of the various spectral components, and indeed prevents on occasion the actual detection of the severely absorbed spectral components.